A notorious enemy spy falls for an American medical student during World War I.

Annemarie, a seemingly deluded patient of a convent asylum, remembers her past while waiting for a mysterious man to fetch her: In Berlin of 1915, master spy Annemarie, known in the field as Fräulein Doktor, returns from a successful assignment and reports to Von Sturm, the head of the German secret service. After Annemarie advises him to be wary of Mata Hari, who has made the mistake of falling in love with her enemy, Von Sturm tells her about Ali Bey, the commander of the Turkish Dardanelles, who he suspects is a British spy. Von Sturm assigns Kruger, one of his agents, to go to Constantinople and approach Ali Bey as a fellow British spy. Before Kruger leaves Berlin, however, Annemarie follows him to a dentist's office, where he reveals himself to be an English spy trying to pass information to an equally traitorous dentist. During the raid on the dentist's office, Douglas Beall, an American medical student, is arrested and questioned by Von Sturm. Although Douglas eventually is released, Von Sturm asks Annemarie to investigate him further. That night, the skillful Annemarie, now calling herself Helena Bohlen, tricks Douglas into picking her up at a restaurant. Taken with the beautiful spy, the unsuspecting Douglas confesses his love to her in his hotel apartment and is dismayed when she slips away without saying goodbye. However, when Annemarie, who has been given Kruger's assignment, heads for the Constantinople-bound train, she finds Douglas in her taxi. In spite of her protests, Douglas boards the train with Annemarie and stays as close to her as possible. Soon after, the Allies bomb the train, which is carrying German soldiers, and Douglas impresses Annemarie by treating the injured passengers. Now in love with Douglas, Annemarie orders her fellow spy Karl, who is posing as her secretary, to return to Berlin and helps Douglas to enter Turkey on her passport. In Constantinople, Annemarie arranges to meet Ali Bey at a charity bazaar and immediately dazzles him with her aloof charm and beauty. Later Annemarie tells Ali Bey, who has been warned by telegram that the infamous Fräulein Doktor is in the city, that she is his British contact. Later, Annemarie confesses to a jealous Douglas her mission in Turkey and, in spite of his ultimatum that she leave immediately with him or risk their relationship, remains in Turkey to expose Ali Bey. Concerned for Douglas' safety, however, Annemarie arranges for Karl to pick up the American. She then orders Von Sturm to fake Douglas' execution in front of her and Ali Bey, confident that Ali Bey will assume that she is sacrificing her love to keep her identity as a British spy a secret from her superior. The ploy works, and using invisible ink, Ali Bey writes strategic information about German submarine movements on Annemarie's shoulder, then is arrested by Von Strum. Although Douglas was secretly transported out of Turkey, Von Sturm tells Annemarie that he was executed to keep her from quitting her espionage work. Overcome by grief and guilt, however, Annemarie lapses into a state of hopeless delusion. Back in the convent, Annemarie's dream that Douglas will one day find her finally comes true, and the couple happily reunites.